Jews are not superstitious, period. They just believe that knocking on wood may save them from impending doom. And that's just good logic.
The most favorite Jewish superstition is spitting three times, noisily as possible, after discussing something horrible. For Example: "Did you hear that Yafa was run over by a steamroller? Tfu tfu tfu!!!" The "mehadrin" (or the strictly superstitious) combine spitting and knocking on wood, just to stay on the safe side.
Afraid of breaking glass? Many Jews believe that shattering plates or glasses is a sign of good luck. Thus, you can hear in many Israeli restaurants the sound of some poor waiter dropping a tray, followed by cheering and clapping. Luck, you see, always needs encouraging…
"You don't need all this fancy-shmansy medicine," said my grandma, who always believed modern medicine was a shrewd gentile way of poisoning us all. You want medicine? Take some TCG: Tea, chicken-soup and Gogel-mogel (which is a drink made of milk and honey). And some castor oil can't hurt, either.
If, God forbid, something bad does happen, we try to think back and discover the guilty party. Did you drive on Shabbat, again? Did you not fast on Yom Kippur? When was the last time you went to synagogue, anyway? You'll be the death of us all!
The most frightening superstition is The Evil Eye. Had something bad happen to you? Better check if you wronged someone, and he put the evil eye on you. And yes, anyone can do it. Just give your dirtiest look, and you're hexing away!
When my first child was born, Grandma immediately made me promise "not to show him to strangers." Better even not to talk about him. Why? Well, I'd tell you, but I'm kind of scared of the evil eye… tfu tfu tfu!!!